why was Julius Caesar a traitor?

Answer 1

Julius Caesar had formed a triumvirate with Crassus and Pompey however, on returning with his army from Gaul, he crossed the Rubicon and threatened to take Rome by force.

He was therefore a traitor to Rome because he sparked a civil war.

Answer 2

He had from the beginning denied that he wanted to be Emperor and then suddenly he wanted to be Emperor and he was fooling around with Cleopatra and left his Roman wife at home. The Romans were tired of his two faced politics